1. Field of the Invention
The present invention relates to a speed change control method for an automotive automatic transmission.
2. Description of the Prior Art
An automotive automatic transmission is generally provided with a controller for determining, in accordance with a shift pattern set beforehand, a gearshift position suitable to a running condition of a vehicle, based on detected pieces of information such as throttle opening, vehicle speed, and a gearshift position currently established. The transmission is operable to automatically establish an optimum gearshift position under the control of the controller. In a typical shift pattern, a group of upshift lines for upshift operations from lower gearshift positions to higher gearshift positions, and a group of downshift lines for downshift operations from higher gearshift positions to lower gearshift positions, are set as a function of throttle opening and vehicle speed, as exemplarily shown in FIGS. 1 and 2. When the vehicle running condition changes from a region indicated by numeral "1" in FIG. 1 to a region indicated by numeral "2," for instance, an upshift operation from the first speed to the second speed takes place. Also, a downshift operation from the fifth speed to the fourth speed takes place when the vehicle running condition changes from a region indicated by numeral "5" to a region indicated by numeral "4," for instance. Likewise, an upshift or downshift operation between other regions also takes place.
In this manner, the shift pattern is set such that a gearshift position for higher speed is selected at a lower vehicle speed region if the throttle opening is small, whereas a gearshift position for lower speed is selected in a higher vehicle speed region if the throttle opening is large. Thus, even if a vehicle runs on a descending road, a gearshift position for higher speed by comparison with vehicle speed is selected when the throttle opening is made small, so as to operate an engine in a lower rotational speed region. That is, an upshift can take place when the throttle is restored during the vehicle running on a descending road. Thus, engine braking sometimes does not to work effectively when the vehicle runs on a descending road. In this case, the vehicle speed increases, and hence a braking force is needed, so that a driver is required to operate a foot brake, or operate a select lever for switching drive ranges so as to cause engine braking to be exercised.
To obviate such a drawback, an automatic transmission has been proposed, in which a downshift takes place when it is determined that a vehicle runs on a descending road. Such a determination is made by comparing gradient resistance to the vehicle derived from detected information with a determination reference value.
According to the proposed automatic transmission, however, since the determination reference value for the determination of vehicle running on a descending road is fixed, a downshift takes place uniformly when such a descending running determination condition is satisfied. Thus, application of engine braking is always initiated upon fulfillment of the determination condition. On the other hand, a vehicle running condition in which engine braking for vehicle travel on a descending road should be exercised varies depending on drivers. Thus, it is difficult to satisfy a preference of an individual driver by means of such a uniform downshift control.
As another countermeasure for eliminating the need of a foot brake operation or a select lever operation during the vehicle travel on descending roads, a fuzzy speed change control based on vehicle speed, road gradient, etc., indicative of a vehicle travel condition, has been proposed. The fuzzy speed change control makes it possible to automatically effect a downshift so as to exercise engine braking when a vehicle runs on descending roads.
However, according to a conventional fuzzy speed change control designed to conduct fuzzy inference for shift pattern selection directly from variables indicative of a vehicle running condition, such as gradient and vehicle speed, fuzzy rules must be set for all the combinations of the vehicle running condition variables, to make it possible to select an optimum shift pattern even if a vehicle runs in any condition. In addition, optimally setting a large number of fuzzy rules is very difficult. On the other hand, if fuzzy rules solely for typical running conditions are set, a shift pattern selection can be made inappropriately depending on the vehicle running condition.
As mentioned above, a shift pattern includes an upshift line group and a downshift line group which are set as a function of throttle opening and vehicle speed. In this respect, if a speed change control is conducted in accordance with a single shift pattern, then an upshift or a downshift takes place uniformly each time an operating point represented by throttle opening and vehicle speed crosses an upshift line or a downshift line. This sometimes makes it difficult to perform a speed change operation suitable to vehicle driving on ascending roads or a speed change control demanded by a driver.
In order to achieve a speed change control suitable to vehicle driving on ascending roads, a speed change control apparatus has been proposed in which a shift pattern for flat roads and a shift pattern for ascending roads, which makes an upshift difficult to take place, are set beforehand. Further, a vehicle running on an ascending road is determined to select the shift pattern for ascending roads, if several variables, such as vehicle speed and gradient, indicative of the vehicle running condition satisfy a particular conditions. This thereby effects a speed change operation suitable to the vehicle running on ascending roads, while preventing an unnecessary upshift.
However, in the proposed speed change control apparatus, the same shift pattern for ascending roads is used for both of a steep ascending road and a gentle ascending road. Depending on the gradient or the vehicle speed, this can prevent an upshift even if a driving force is sufficient, and can permit an upshift even if a driving force is deficient, resulting in shift hunting.
Further, to achieve a speed change operation demanded by a driver, a speed change control apparatus has been proposed in Japanese Provisional Patent Publication No. 2-275174, in which an ordinary shift pattern (normal pattern or economy pattern), and a sporty pattern (power pattern) where shift lines are provided on the higher speed side than those of the normal pattern are set beforehand, and in which an arbitrary one of these patterns is selectable manually or automatically. According to a speed change control apparatus of this kind, changeover from the normal pattern to the sporty pattern automatically takes place when an accelerator depression speed exceeds a threshold value, which varies dependent upon the vehicle speed and the throttle opening, under a condition where predetermined requirements such that a select lever is in a D-range are fulfilled.
The threshold value of the accelerator depression speed is set, as exemplarily shown in FIG. 3. In the case of this setting example, the entire vehicle running area is divided into sixteen regions by use of throttle opening and vehicle speed as parameters, and threshold values T11 through T44 for the respective regions are set. The threshold values T11 through T44 are set such that one for a higher vehicle speed region has a larger value than one for a lower vehicle speed region (T11&lt;T12&lt;T13&lt;T14, for instance), so that the switching to the sporty pattern is likely to take place at a lower vehicle speed. Further, the threshold values T11 through T44 are set such that one for a larger throttle opening region has a smaller value than one for a smaller throttle opening region (T11&gt;T21&gt;T31&gt;T41, for instance), so that the switching to the sporty pattern is likely to take place at a larger throttle opening. As a consequence, a shift from the normal pattern shown by solid lines in FIGS. 1 and 2 to the sporty pattern shown by doted lines takes place when the accelerator pedal is depressed at a speed higher than an associated threshold value (T11, - - - , or T44).
However, the proposed apparatus, which only permits selection of either one of the two shift patterns, cannot achieve an elaborated speed change operation which fully meets a vehicle running condition or a driver's demand.
As another prior art for shift pattern change, it has been also known to set a plurality of, e.g., five upshift characteristic lines SC1 through SC5 in connection with an upshift from the third speed to the fourth speed, for instance, as exemplarily shown in FIG. 4, and select one of these shift characteristics in accordance with a vehicle running condition represented by throttle opening, engine rotational speed, vehicle speed, lateral acceleration, and longitudinal acceleration, for instance.
However, using a large number of shift patterns in this manner requires a lot of labor in setting the shift patterns, and takes a lot of time for determination processing to select an optimum pattern from the shift patterns which is large in number, causing a delay in shift control to a change in the vehicle running condition.
Further, to eliminate drawbacks produced when a speed change control is effected with use of a conventional shift schedule which requires manual speed change from the D-range to the 2nd-range or a brake pedal depressing operation to compensate a deficiency in engine braking performance which occurs when a vehicle runs on descending roads, it has been proposed in Japanese Patent Publication No. 52-20630 to use a particular shift schedule including a specific speed region, corresponding to a nearly fully closed throttle opening, for automatic speed change, the specific speed region being expanded toward a predetermined vehicle speed which is usually included in an adjacent speed region for which a gearshift position which is one level higher than that for the specific speed region is established (see, FIG. 47).
However, In case that a downshift line for a small throttle opening region is set on the higher speed side, as in this proposal, even if a driver wishes to make a coasting vehicle running, a downshift takes place contrary to the driver's intention, to cause engine braking to be exercised. This causes the driver to have a feeling of dissatisfaction or incompatibility.
On the other hand, in case that a downshift line for a small throttle opening region is set on the lower speed side, as distinct from the aforementioned proposed shift schedule, a downshift does not take place as long as the accelerator pedal is not depressed by the driver. Thus, a delay can be caused in downshift, e.g., when the vehicle passes a sharp corner in sporty driving during which the driver makes braking at a location short of the corner and then depresses the accelerator pedal for re-acceleration. That is, the downshift does not take place at the location short of the corner, but takes place when the vehicle reaches the exit of the corner. In this case, acceleration at the exit of the corner is prevented by the downshift effected at the corner exit.
As explained in the above, it is difficult heretofore to achieve an elaborated speed change control with the use of a relatively simplified control system.